Interesting thread because I never considered the difference in having large or small breasts.
My issue about bras has always been sexuality at work: as a woman you either have to feel super-confident enough to brandish your sexuality or to keep it off the agenda. The comment about underwired jock straps made me giggle. I go bra-less often but didn't like my nipples showing at work or in other situations where I feel sexually vulnerable.
It is a sexuality issue for me. Leading on from that, I wonder how relaxed you women feel about sun-bathing topless. It's normal in several places that I've lived, and makes me feel good to see other women doing this. The whole thing about "covering up our bits"...